Fresh water is becoming a limiting resource as a result of human activity. We are both consuming more, due to our increasing population, and simultaneously reducing availability, due to pollution and the anthropogenic contribution to climate change. It is imperative that we develop strategies to utilise water resources more efficiently.
In a domestic environment people use water for drinking, cooking, washing, sanitation and irrigation. This water comes from many sources including rainwater tanks, wells, lakes, rivers and streams. It is often polished to remove contaminants. In many countries this water is supplied through centralised municipal systems where the water is extensively treated to be suitable for drinking. As these resources increasingly fail to meet demand, sea water and, more recently treated sewage are being utilised to fill the gap.
The supply of a high quality water resource has been a boon for human health, but the large scale transport and treatment of water has generated vast quantities of greenhouse gases, emitted during production of the energy required for these processes, which has amplified the shortage of fresh water.
Less than 2% of the municipal water supplied is used for drinking, so high quality water resources could be spared by supplying lower quality water for irrigation, toilet flushing and laundry. These needs could be met by rainwater or treated waste water. Currently, most of this non-potable water is generated in municipal facilities where sewage (black water) is treated to a standard suitable for non-potable uses. The process of transporting, treating and returning the treated water from large centralised facilities has proved expensive in terms of energy consumption and infrastructure development, so methods have been explored to enable local treatment of the waste water. Local treatment has the added benefit of reducing risks associated with the spread of infectious agents.
The largest portion of domestic waste water, by volume, is produced by washing of people and their clothes and this waste water is often called grey water. By separating these sources from the much more heavily contaminated toilet and kitchen waste it should be possible to develop energy efficient systems that can produce water suitable for toilet flushing, irrigation, laundry and car washing.
At its most basic, such a system would capture domestic grey water, from washing clothes and bathing, and divert it through coarse filtration to subsurface irrigation, where the soils are suitable and rainfall is minimal. The water remains contaminated with infectious agents, so must not come into contact with people or their food. It also contains particulate, like hair and lint, and organic material, which encourages bacterial growth in the filters and the irrigation networks. The accumulated hair and lint together with bacterial growth, causes clogging of filters and the irrigation network: necessitating frequent maintenance and exposure of people to untreated waste water. This basic system does not therefore maximise the reuse potential of the waste water and presents a health risk to those maintaining the systems.
More complex systems have been described that filter and sanitise the grey water, most commonly with chlorine, ozone or ultraviolet light. After this level of treatment the water is suitable for toilet flushing and irrigation, including food crops. However as the water still contains organic material, bacterial growth still occurs in and around the irrigation networks and toilet cisterns. Once again the true potential for reuse of the waste water is not exploited and system maintenance still presents a health risk.
Systems have also been described that treat the grey water to remove the organic material as well as the particulate material and infectious agents. In PCT/AU2008/000213 a combination of flocculation with aluminium sulphate, sanitation with calcium hypochlorite and filtration is used to treat laundry waste to a very high standard. However systems like this, which produce water treated suitable for toilet flushing and irrigation and do not expose the user to infectious agents, require regular addition of chemicals and periodic replacement of filters. This does increase the cost of operation and precludes the use of such systems in developing countries.
Other systems have been described that use biological processes and these can provide high quality water but suffer from three limitations. Firstly, they require a critical mass of viable bacteria to degrade the organic material. As a result they cannot generate treated water immediately upon supply of waste water. Secondly, the bacteria responsible for degrading the organic compounds are sensitive to the presence of biocides in the waste water which in the domestic environment can include hypochlorites, QUATs, hydrogen peroxide, percarbonates, cyclohexidine and stabilised chlorine dioxide. Thirdly, the bacteria responsible for degrading the organic compounds must be removed from the waste water by some form of filtration, which can be difficult to maintain. Generally these systems require the home owner to modify their waste disposal behaviour and follow detailed guidelines on when their treated water is safe fir reuse as there is no easy way for them to know when such a system is fully treating the waste water. To circumvent these problems non-biological treatment steps have been added to these systems such as the use of ozonation in PCT/AU2009/000553, but these systems have proved complex to control, expensive to construct and difficult to maintain, so have not proved popular even in developed nations.
There is therefore a pressing need for a system that ameliorates the problems mentioned above, or at the very least provides an alternative to currently known systems. Such a system would ideally be mechanically simple and not require regular replacement of chemicals or filters, yet be functionally robust and capable of treating waste water locally to improve the efficiency of water use in developed and developing countries.